Multiple visual areas exist in mammalian cerebral cortex. The purpose of this project is to determine, in human beings and trained monkeys, both the immediate and long-term effects upon visual tracking of lesions in specific "visual" areas and pathways. Smooth pursuit with the head still and with the head moving (VOR cancellation), optokinetic nystagmus and saccadic tracking will be examined in monkeys with ablations of: 1) cerebral cortex in one hemisphere, 2) unilateral and bilateral regions of parietal-temporal-occipital cortex, and lesions in the 3) retino-geniculo-striate pathway and 4) retino-tectal-pulvinar pathway. Selected patients who have lesions in these regions will be similarly examined. Eye movements will be measured using the magnetic field search coil technique. "Open loop" performance, a more sensitive measure of pursuit capabilities, will be measured either during the initiation phase of tracking before visual feedback can help, or by using an artificial feedback technique to open the visual feedback loop. The oculomotor behavior of our monkeys and patients will be compared to simulated lesions in models of visual tracking. Monkeys and patients with visual tracking deficits will also be examined for latent and spontaneous nystagmus (ocular motor disorders attributed to visual tracking abnormalities). A more complete understanding of the anatomy and physiology underlying visual tracking will help clinicians to understand better the significance of abnormal visual tracking in patients, as well as a number of human oculomotor disorders which are attributed to visual tracking abnormalities.